This invention relates to the manufacture of incandescent lamps and, in particular, to providing a light-diffusing coating on the bulb by electrostatic deposition.
As is known, it has long been desired to produce softer light from an incandescent source, i.e., produce less distinct shadows or have it seem that the light is produced by a distributed rather than point source. A solution to this problem was obtained by etching the inside of the bulb with acid. Later silica powder was used to coat the inside of the etched bulb, which further softened the light from the filament.
Neither of these approaches is entirely free from difficulty. For example, the acid etch slightly weakens the glass and the acid poses a materials handling problem.
A proposed alternative has been to electrostatically deposit silica particles to obtain a diffusion coating. One example of this is disclosed in U.S. Pat. No. 2,922,065, Meister et al. Meister et al are primarily concerned with lumen maintenance and disclose an electrostatically deposited, controllably hydroscopic layer of extremely fine (0.02-1.mu.) silica particles. The hydroscopic characteristic is obtained by heating the silica particles to no more than 625.degree. C to preserve a number of hydroxyl sites on the surface of the powder to which water molecules can later attach. The lack of these sites, produced by high firing temperatures, is believed to be the "inertness" referred to by Meister et al.
Lumen maintenance aside, the basic problem is to provide a diffusion coating that adequately diffuses the light and hides the filament without greatly decreasing lumen output. These, in turn, derive from the more basic operations of powder deposition and coating adherence, i.e., the coating must be sufficiently, but not overly, thick and must adhere during further manufacturing operations and during use. Prior art attempts at improving powder deposition and uniformity have been concerned with apparatus, such as U.S. Pat. Nos. 2,806,444 and 3,126,300, or with post-deposition treatment, such as U.S. Pat. Nos. 2,811,131 (steaming the coating to compact it) or 2,995,463 (using a follow-up electrostatic field to increase adherence), or with powder additives to prevent agglomeration as in U.S. Pat. No. 2,878,136. So far as is known, none are concerned with the powder and the deposition process per se.